This chapter is from the book

Explain how computers can communicate binary 1s and 0s using
electricity

List the components of popular LAN cables and connectors

Compare and contrast straight-through and cross-over Ethernet
cables

Explain the operation of an Ethernet hub

Summarize the benefits of using a structured cabling system

So far in this book, you have read about how networks allow computers to
communicate. Networks include software, some of which sits in the computers, and
some of which sits in routers. The network also includes hardware, such as the
network interface cards (NICs) in the PCs, mentioned in Chapter 3,
"Building a Network: It All Starts with a Plan." Finally, networks
include cabling, which provide a physical means to transmit bits across a
network.

This chapter is the first one in this book's second major part:
"Running the Local Department of (Network) Transportation." In the
United States, most cities, all states, and the U.S. federal government have a
department of transportation (DOT). Each DOT plans, builds, and fixes problems
with roadways. You can learn a lot about networking by comparing networks to
roads.

Driving Bits Across the Network Roadway

I just got back from lunch at one of my favorite lunch places: La Frontera
Mex-Mex Grill. When it was time for lunch, I got in my car, drove to the
restaurant, ordered my usualnumero dos, con pollo, por favorand
drove back home.

When I got home, my wife said, "So you drove over that really cool
street outside our house on your way to lunch, huh?" Yeah, right. Who cares
what roads I drove on? It's where I drive to that's
important.

Well, to know networking well, you need to know some of the basics about the
networking equivalent of roads. If you find yourself really getting interested
in what's in the next few pages, you might just be one of those people who
really would like a career working with the technical side of networking. If
not, you should at least know the concepts so that you can communicate with the
networking geeks of the world and have a firm understanding of networking.

What's a Local-Area Network?

Chapter 3 defined a local-area network (LAN) as a network in which the
devices are relatively close together. Of course, a network, once again
according to this book, includes computers, software, hardware, and cabling that
allow the computers to communicate. Although this definition is accurate, you
really do not get a detailed picture of a LAN this way. So, Figure 4-1 shows a
LAN, which is one you've seen before in Chapter 3.

In this case, the LAN consists of some obvious elements. First, you need at
least two computers. The computers need to have networking software; otherwise,
they will never attempt to communicate. They also need the physical ability to
transmit bits from one computer to the otherhence the need for the
cable and the NICs in each computer. (Figure 4-1 shows the NICs
outside the PCs so that you can see them, but they are typically inside the
PCs.)

LANs do not get any simpler than this one. Larger LANs can get much more
complex, with lots more components like networking hubs and switches, which you
will learn more about in the next few chapters.

In this chapter, you learn about how computers can transfer bits across the
network roadway. The topics covered here might be the equivalent of what a DOT
engineer might talk about over lunch with a stranger: "Hey, you know
we're going to be paving Parker-Puckett Parkway pretty soon. Pretty cool,
huh?" The reply, "Hey, isn't that near that Mexican place, La
Frontera?" And the response from a true DOT engineer: "I don't
knowdon't care. That's beside the road. I just care about
the road." Likewise, some network engineers think of the LAN as the cable,
possibly the NICs, but they typically don't care a lot about the computers
that happen to be connected to the cabling. Likewise, this chapter focuses more
on how computers send bits to each other, rather than the applications that run
on those computers.

Transmitting Bits Across the Local Network Roadway

Back in Chapter 2, "A Network's Reason for Existence," you saw
an example where Fred opened a file that sat on Wilma's disk drive. Later,
Fred printed the file on the printer connected to Wilma's computer, and
finally, Fred saved the file back on Wilma's disk drive. Figure 4-2 shows
the process, in sequence.

For this process to work, Fred and Wilma must be able to cause a bunch
of bits that sit in memory in one computer to be sent to the other
computer. A file is just a bunch of bits. So, imagine that by using a word
processing program on a single computer, you can open a file, read the contents,
and edit and change the file. Similarly, if you can send the same bits in that
file to another computer across the network, you or someone else can edit the
file by using the same word processing program on that computer. So, for a
network to work, the network needs to be able to get a bunch of bits from one
computer to another.

Driving Bits Across a Wire

To send one binary code from one device to another, the sending device puts
some electricity on the wire. Electrical signals have many characteristics that
a NIC can control and vary. By varying one of these features to two different
values, with one value meaning binary 1 and one meaning binary 0, you can
transfer data over the wire.

For example, imagine that both PC1 and PC2 have a NIC, and there is a single
wire connecting the two cards. The wire is just a skinny, long piece of copper,
and copper conducts electricity very well. Now, imagine that the encoding
standard used by the company that made the NIC defines that a binary 0
is represented by a voltage of ±5 volts, and a binary 1 is represented with ±10
volts. Encoding is the term that refers to a set of rules that defines what a
sender should make the electrical signal look like to imply a binary 0 or a
binary 1. Figure 4-3 depicts the general idea.

In the figure, PC2 generates some electricity on the wire. In this case, PC2
wants to send the binary value 0101. So, it sends a 5-volt signal, then 10
volts, then 5 volts, and then 10, because the imaginary encoding scheme in this
example states that 5 volts means 0, and 10 volts means 1. PC1, on the other end
of the wire, senses the incoming electrical signal and interprets the
electricity, using that same set of encoding rules to mean 0101, exactly as PC2
intended.

Note that the graph shown in Figure 4-3 shows a discrete, or constant,
voltage. Because the X-axis (horizontal axis) represents time, when the voltage
changes, it changes immediately to the next value. The use of discrete, constant
values, which are then instantly changed to other possible discrete values (as
in Figure 4-3), is called digital transmission. To transmit binary
numbers, or binary digits, it is useful to transmit the data using
digital transmission.

For the digital transmission of data to work correctly, not only must the
sender and receiver agree to what electrical characteristics mean a binary 0 or
1, but they also must agree to the rate at which the bits are transmitted over
the wire. In Figure 4-3, the receiver (PC1) must think about the electrical
signal at different points in time, on a regular interval. Likewise, the sender
(PC2) must use this same regular time interval to decide when it should change
the digital electrical signal. For instance, if PC2 varied the voltage to mean
either 0 or 1 every .1 seconds, and PC1 sampled the incoming electrical signal
every .1 seconds, they could transfer 10 bits in a second. The speed of this
network connection would be 10 bits per second.

If the two PCs did not agree on the transmission speed, the devices
couldn't transfer the binary information. For instance, imagine that PC2
thought the speed was 10 bits per second, meaning it should encode a new bit
every 1/10 of a second. If PC1 thought that it should be receiving a bit 20
times per second, it would sample the incoming electrical signal every 1/20 of a
second. PC1 would think it was sending 10 bits each second, and PC2 would think
it received 20 bits.

The term bps (short for bits per second) often refers to the
speed of networking connections. Note that the unit is bits, not bytes. In real
life, LANs typically run at much higher speeds, with a slow LAN transmitting at
10 million bits per second (Mbps, also called megabits per second).

Notice that Figure 4-3 represents electricity as a square
waveform, with positive and negative voltages. You don't
really need to worry about the electrical details, but as you progress through
learning about networking, you will see other drawings like this one. The
networking cards use an alternating current, or AC. The positive voltage means
the current is in one direction, and the negative current means the current runs
in the opposite direction.

The Need for a Two-Lane (Network) Road

In Figure 4-3, PC2 sends an electrical signal to PC1. As it turns out, if PC1
tried to send some electricity to PC2 at the same time, over the same wire, the
electrical signals would overlap, and neither PC1 nor PC2 could understand what
was sent.

To solve the problem, PC1 and PC2 need to use two wiresone for PC1 to
send bits to PC2, and one for PC2 to send to PC1. Figure 4-4 shows two wires,
with PC1 and PC2 sending and receiving at the same time.

The Equivalent of Asphalt: Cables

You can get in your car and drive around your yard, the sidewalk, in your
neighbor's yard, or through the park. Of course, it's better if you
drive on the road! The earlier examples in this chapter showed NICs using a
single copper wire for data transmission in each direction. But rather than just
have a couple of wires somehow stuck into the side of the cards, in real life,
we use cabling and connectors to manage the wires, making the job of the
electrician much more comfortable and convenient.

The copper wires that networking cards use are encased inside a cable. The
cable is made from plastic, with the copper wires inside the cable. Figure 4-5
shows a drawing of the most popular type of cabling used for LANs today.

If you look closely at this figure, you can see each copper wire, as well as
the plastic coating on the wire. The copper wire is thin, making it brittle. In
fact, the wire could easily break in your hand. To help prevent the wire from
breaking, a thin plastic coating is painted onto each wire. Conveniently, each
wire uses a different color of plastic coating, so you can look at each end of
the cable and figure out which wire is which. As you might guess, and as you
will learn more about in the next few pages, it is important that you can
identify a particular wire on each end of the cable.

Also note that the wires in Figure 4-5 are twisted together in pairs. Each
pair of wires is cleverly called a twisted pair. The term refers
to a pair of wires twisted around each other to reduce the amount of electrical
interface on the wires. In layman's terms, electromagnetic
interference (EMI) occurs when electrical signals that exist in the
aircaused by other wires or other nearby electrically powered
deviceschange the electrical currents on the wire. If outside EMI changes
the signal on the wire, the receiving computer might misinterpret a 0 as a 1 or
a 1 as a 0, or it might not have a clue what the sender really sent. Sending the
electrical signals over a twisted pair rather than a single wire eliminates a
lot of EMI effects. (Besides, the wires are pretty skinny anyway, and copper is
cheap, so why not use two?)

Another thing that can be added to the cable to reduce EMI is shielding.
Shielding, as the name implies, shields the wires inside the cable from the
effects of EMI. However, shielded cabling has more stuff in it, making the cable
less bendable and more expensive to produce. Shielded cables are called
shielded twisted pair (STP), and you could probably guess that
unshielded cables are called unshielded twisted pair (UTP). LAN
technology has evolved to the point where less expensive UTP cabling can be used
in most environments, with STP cabling being used in environments where
significant EMI issues exist. Figure 4-5 showed UTP cabling; Figure 4-6 shows an
example of an STP cable.

Painting the Lines on the Road: Connectors

Imagine that you see a new road that has just been paved. The road is so new
that the DOT hasn't even painted the lines yet. So, you turn onto the road
and enjoy the ride. After a couple of miles, you look up and see that someone
else is on the roada huge, speeding truckand the driver wants to
take his half of the road in the middle of the road. After swerving off the
road, you might think to yourself, "Boy, that reminds me of one of the
reasons that you use connectors on the end of networking cables!"

Okay, that's far fetched, but it does lead into a key point about
connectors. If roads had no lanes, and there were no traffic laws, the roads
would be pretty dangerous so the DOT paints lines on the road to create traffic
lanes. Similarly, connectors line up the wires on the end of a cable into
well-defined physical locations inside the connector. Essentially, each wire in
the cable is identified by color, and each colored wire has a specific reserved
place inside the connector that the electrician attaches to the end of the
cable. The connectors put the wires into the right place, just like the lines on
a road guide cars into the right place.

An electrician can take a cable and attach a connector to the end of it. When
he attaches the cable to a connector, each of the wires protrudes into the
connector so that the electricity can flow when connected to a device. The tip
of the exposed wire in the connector is called a pin. A pin is
nothing more than a physical position in the end of the connector in which the
copper part of the wire sits. You can think of a pin like you think of a lane on
a road. Figure 4-7 shows a photo of a typical connector, called an RJ-45
connector, along with a drawing of the same connector.

The Telecommunications Industry Association (TIA) and Electrical Industries
Alliance (EIA) define standards for cables and connectors. For instance, they
define how to use an RJ-45 connector for LANs and options for several types of
UTP cabling. (You can learn more about TIA and EIA by going to their websites,
at http://www.tiaonline.org
and http://www.eia.org.)

If you have never seen an RJ-45 connector, and you use Ethernet LANs at work
or school, you could remove the cable from your PC's network card and look
at it. In most cases, the cable will be using an RJ-45 connector. (Ethernet is
by far the most popular type of LAN today. You'll read more about it in the
next several chapters.)

The EIA/TIA defines standards for which wires fit into which pins when you
make a cable for use with Ethernet. Two of those standards are shown in Figure
4-8.

Each of the drawings in the figure represents an RJ-45 connector. The RJ-45
connector allows eight wires to be inserted into it. EIA/TIA standards suggest
the numbering schemes for the eight pin locations and the pairs of wires,
knowing that a twisted pair is needed for data transmission. The standard also
specifies which color of wire goes into pin position 1, 2, and so on.

The RJ-45 connectors have clips on the side that allow you to easily insert
the connector into the plug (hole) in the networking card in a computer. At that
point, the connector is secure and should stay put. If you push the clip close
to the rest of the connector, it releases the connector from the card. So, the
clip helps keep the connector secure and allows you to pull the connector out
when you are ready.

So, why does all this matter? In the next few pages, it will all come
together as you see how NICs try to use the cables and wires.

Driving in the Right Lane (Pair) on the Road

Refer to the simple network of Figure 4-1 early in this chapter. That network
consisted of two PCs, each with a networking card, and a cable between them. So
far, you've learned the basics about wires, cables, and connectors.
However, there's one last important thing about basic LAN data transmission
that you need to know, and it relates to which wires are used for actual data
transmission by Ethernet LANs.

Ethernet NICs in PCs try to send data over the twisted pair that uses pins 1
and 2 of an RJ-45 connector. These same NICs expect to receive data on the
twisted pair that uses pins 3 and 6. However, without the right kind of cabling,
two Ethernet NICs cannot communicate. Figure 4-9 depicts the effect when both
NICs try to send using the twisted pair that uses pins 1 and 2.

(Rather than clutter the figure with more cabling, I just drew the two pairs
of wires; the wires do indeed sit inside a single cable, with RJ-45 connectors
on each end.)

The cable in Figure 4-9 puts one end of a wire in pin 1 of one connector, and
the other end into pin 1 of the other connector. Pin 2 on one end of the cable
connects to pin 2 on the other side; and so on, for all eight wires. A cable
with the wires connected in this manner is called a straight-through
cable.

Okay, back to the problem illustrated in Figure 4-9. As you can see from the
bubbles in the figure, both PC's NICs send on the twisted pair at pins 1
and 2. That electricity goes over the wires and enters the other NIC on pins 1
and 2. But, the NICs aren't receiving data on pins 1 and 2! That's
because Ethernet NICs try to send on pins 1 and 2, and they receive data on the
pair at pins 3 and 6. In this case, both PCs send, but neither receives
data.

The solution is to use a cross-over cable. Cross-over cables
connect the wire at pin 1 on one end of the cable with pin 3 on the
other end; the wire at pin 2 with pin 6 on the other end; the wire at pin 3 with
pin 1 at the other side; and the wire at pin 6 with pin 2 at the other side. The
result: The PCs can receive the data sent by the other device! Figure 4-10 shows
the basic idea.

Now you know the basics of how you can allow two PCs to attach to a cable and
transfer bits between each other. This chapter focuses on how to build the local
network roadwayessentially, the networking components that allow bits to
be transferred. Next, you'll read about how to connect several devices on
an Ethernet LAN, using a device called a hub.